Concerning a conventional antenna device, an example of reception antenna device is explained below by referring to FIG. 7 and FIG. 8.
FIG. 7 is a block diagram of a conventional antenna device. The conventional antenna device comprises, as shown in FIG. 7, an antenna element 101 having a length of about a quarter wavelength of reception wave, a matching circuit 102 connected to this antenna element 101 and formed of a reactance element, and an output terminal 103 connected to the output of this matching circuit 102.
In such conventional structure, however, if an antenna element 101 of a sufficiently small length as compared with the wavelength is used in order to realize a small-sized antenna device, the resistance component of the output impedance of the antenna element 101 is about 0 ohm. It was hence very difficult to match it by using the matching circuit 102 composed of reactance.
This problem is explained together with the Smith chart in FIG. 8. On the Smith chart, as shown in FIG. 8, impedance value 104 at 90 MHz and impedance value 105 at 108 MHz of the antenna element 101 sufficiently small as compared with wavelength are far from target impedance 106 of the output terminal 103, that is, 75 ohms. It is hence necessary to bring the impedance value 104 and impedance value 105 closer to the target impedance 106 of 75 ohms of the output terminal 103 by means of the reactance element of the matching circuit 102.
At this time, to move a long distance from the outer circumference of the Smith chart to the center, the reactance value of the matching circuit 102 is increased. Assuming an inductor as reactance element, however, when the inductance of the inductor is increased, reactance value 107 at 90 MHz and reactance value 108 at 108 MHz are largely different from each other, and the distance between the two is considerably longer than the initial distance between the impedance value 104 at 90 MHz and impedance value 105 at 108 MHz. That is, the impedance variation due to reception frequency increases.
To avoid such problems, hitherto, the antenna element 101 was defined at quarter wavelength of reception wave. Thus, matching with the matching circuit 102 is easy, but the size is increased. For example, at 100 MHz, the wavelength is 3 m, or at 1 GHz, it is 30 cm.